Carrier tracking loop lock detector

ABSTRACT

An ATSC (Advanced Television Systems Committee) VSB (Vestigial Sideband) receiver includes a carrier tracking loop (CTL) for processing a received ATSC VSB signal and a CTL lock detector. The CTL lock detector includes an averaging filter for averaging the down-converted received ATSC VSB signal to provide a DC offset, an estimator for providing an estimate of the DC offset, and a comparator for providing a lock signal as a function of a comparison between the estimate of the DC offset and the DC offset, wherein the lock signal is representative of either a locked condition or an unlocked condition of the CTL.

This application is a national phase application and claims the benefit,under 35 U.S.C. § 365 of International Application PCT/US03/35941, filedNov. 12, 2003, which was published in accordance with PCT Article 21(2)on Jun. 10, 2004 in English and which claims the benefit of U.S.provisional patent application Ser. No. 60/429,367, filed Nov. 26, 2002.

BACKGROUND OF THE INVENTION

The present invention generally relates to communications systems and,more particularly, to a receiver.

In the ATSC (Advanced Television Systems Committee) standard for digitalterrestrial television (DTV) in the United States (e.g., see, UnitedStates Advanced Television Systems Committee, “ATSC Digital TelevisionStandard”, Document A/53, Sep. 16, 1995), the modulation system consistsof a suppressed carrier vestigial sideband (VSB) modulation with anadded small in-phase pilot at the suppressed carrier frequency, 11.3 dBbelow the average signal power. An illustrative spectrum for a ATSC VSBsignal is shown in FIG. 1.

A typical ATSC-VSB receiver includes a carrier tracking loop (CTL) thatprocesses a received ATSC VSB signal to both remove any frequencyoffsets between the local oscillator (LO) of the transmitter and LO ofthe receiver and to demodulate the received ATSC VSB signal down tobaseband from an intermediate frequency (IF) or near baseband frequency(e.g., see, United States Advanced Television Systems Committee, “Guideto the Use of the ATSC Digital Television Standard”, Document A/54, Oct.4, 1995; and U.S. Pat. No. 6,233,295 issued May 15, 2001 to Wang,entitled “Segment Sync Recovery Network for an HDTV Receiver”). The CTLgenerally includes: a Hilbert filter and corresponding delay, a complexmultiplier, a phase detector, a first order loop filter, with aproportional plus integrator path, a numeric controlled oscillator (NCO)and a sine-cosine lookup table. Generally, the ATSC receiver must detectwhether the CTL is “locked” or “unlocked” to the received ATSC VSBsignal. For example, if the ATSC receiver detects that the CTL islocked, then the ATSC receiver determines that the received ATSC VSBsignal is “good” and can be used for subsequent recovery of the dataconveyed therein. However, if the ATSC receiver detects that the CTL isunlocked, then the ATSC receiver determines that the received ATSCsignal is “bad” such that portions of the ATSC receiver may then bereset to, e.g., flush out any recovered data now associated with the badreceived ATSC VSB signal, i.e., erroneous data. In addition, after theATSC receiver detects that the CTL is locked, the CTL loop filterparameter may be changed to decrease the loop bandwidth and rejectthermal noise.

Typically, the ATSC receiver determines whether the CTL is locked byusing the loop filter integrator of the CTL. For example, a thresholdvalue is established and if a signal from the loop filter integrator ofthe CTL (the “lock signal”) changes above the threshold value in aspecified amount of time, the CTL is considered unlocked by the ATSCreceiver. Unfortunately, the behavior of the loop filter integrator—andtherefore the lock signal—is affected by impulse noise, thermal noiseand loop bandwidth of the CTL. As a result, erroneous detections oflocked and unlocked conditions may occur. For example, an unlockedcondition may be falsely detected if the threshold is small comparedwith the noise power and loop bandwidth, or a locked condition may befalsely detected if the threshold is large compared with the noise powerand loop bandwidth.

SUMMARY OF THE INVENTION

We have observed that when a carrier tracking loop (CTL) of an ATSC VSBreceiver is actually locked, the carrier pilot present in the receivedATSC VSB signal creates a DC offset in the CTL output signal (thedown-converted received signal). As such, we have realized that this DCoffset can be used to determine the locked condition of the CTL. Inparticular, the DC offset can be recovered by averaging the CTL outputsignal. Further, since the carrier pilot power is known to be 11.3 dBbelow the signal power, an estimated value of what the DC offset shouldbe can be derived from the signal power of the CTL output signal. Assuch, a decision device can then be used to determine whether the CTL isin a locked condition or an unlocked condition as a function of theestimated value for the DC offset and the actual value of the DC offset.Indeed, this technique is applicable to any modulation system for whicha carrier pilot is included in the transmitted signal and the receiverdemodulates a corresponding received signal by down converting thereceived carrier pilot to DC.

Therefore, and in accordance with the principles of the invention, areceiver includes a CTL for down-converting a received signal to providea down-converted received signal and a CTL lock detector for detecting,as a function of the down-converted received signal, whether the CTL isin a locked condition or an unlocked condition.

In an embodiment of the invention, a CTL lock detector averages adown-converted received signal to provide an average signal; estimatesthe average signal from the down-converted received signal; anddetermines whether the CTL is locked or unlocked as a function of theaverage signal and the estimate of the average signal.

In another embodiment of the invention, an ATSC VSB receiver includes acarrier tracking loop (CTL) for processing a received ATSC VSB signal toprovide a down-converted received ATSC VSB signal and a CTL lockdetector for determining if the CTL is in a locked condition or anunlocked condition as a function of the down-converted received ATSC VSBsignal. The CTL lock detector includes an averaging filter for averagingthe down-converted received ATSC VSB signal to provide a DC offset, anestimator for providing an estimate of the DC offset, and a comparatorfor providing a lock signal as a function of a comparison between theestimate of the DC offset and the DC offset, wherein the lock signal isrepresentative of either a locked condition or an unlocked condition ofthe CTL.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustrative ATSC VSB signal spectrum;

FIG. 2 shows an illustrative high-level block diagram of a TV setembodying the principles of the invention;

FIG. 3 shows a portion of a receiver embodying the principles of theinvention;

FIG. 4 shows an illustrative carrier tracking loop for use in thereceiver of FIG. 3;

FIG. 5 shows an embodiment of a lock detector in accordance with theprinciples of the invention;

FIG. 6 shows an illustrative table for use in comparator 140 of FIG. 5;

FIG. 7 shows an illustrative method in accordance with the principles ofthe invention; and

FIGS. 8-10 show other embodiments in accordance with the principles ofthe invention.

DETAILED DESCRIPTION

Other than the inventive concept, the elements shown in the figures arewell known and will not be described in detail. For example, other thanthe inventive concept, a television, and the components thereof, such asa front-end, Hilbert filter, carrier tracking loop, video processor,remote control, etc., are well known and not described in detail herein.In addition, the inventive concept may be implemented using conventionalprogramming techniques, which, as such, will not be described herein.Finally, like-numbers on the figures represent similar elements.

A high-level block diagram of an illustrative television set 10 inaccordance with the principles of the invention is shown in FIG. 2.Television (TV) set 10 includes a receiver 15 and a display 20.Illustratively, receiver 15 is an ATSC-compatible receiver. It should benoted that receiver 15 may also be NTSC (National Television SystemsCommittee)-compatible, i.e., have an NTSC mode of operation and an ATSCmode of operation such that TV set 10 is capable of displaying videocontent from an NTSC broadcast or an ATSC broadcast. For simplicity indescribing the inventive concept, only the ATSC mode of operation isdescribed herein. Receiver 15 receives a broadcast signal 11 (e.g., viaan antenna (not shown)) for processing to recover therefrom, e.g., anHDTV (high definition TV) video signal for application to display 20 forviewing video content thereon. As noted above, and shown in FIG. 1,signal 11 is typically representative of an ATSC VSB broadcast signal.

Turning now to FIG. 3, that relevant portion of receiver 15 inaccordance with the principles of the invention is shown. In particular,receiver 15 includes analog-to-digital converter (ADC) 105, automaticgain control (AGC) 110, band-pass filter (BPF) 115, carrier trackingloop (CTL) 125 and lock detector 200.

Input signal 101 represents a digital VSB modulated signal in accordancewith the above-mentioned “ATSC Digital Television Standard” and iscentered at a specific IF (Intermediate Frequency) of F_(IF) Hertz.Input signal 101 is sampled by ADC 105 for conversion to a sampledsignal, which is then gain controlled by AGC 110. The latter isnoncoherent and is a mixed mode (analog and digital) loop that providesa first level of gain control (prior to carrier tracking), symbol timingand sync detection of the VSB signal included within signal 101. AGC 110basically compares the absolute values of the sampled signal from ADC105 against a predetermined threshold, accumulates the error and feedsthat information, via signal 112, back to the tuner (not shown) for gaincontrol prior to ADC 105. As such, AGC 110 provides a gain controlledsignal 113 to BPF 115, which is centered at the IF frequency (F_(IF))and has a bandwidth equal to 6 MHz (millions of hertz). The outputsignal 116 from BPF 115 is then passed through CTL 125, which is a phaselocked loop that processes signal 116 to down convert the IF signal tobaseband and correct for frequency offsets between the transmitter (notshown) of the broadcast ATSC video carrier and the receiver tuner LocalOscillator (not shown). CTL 125 is a second order loop, which, intheory, allows for frequency offsets to be tracked with no phase error.In practice, phase error is a function of the loop bandwidth, inputphase noise, thermal noise and implementation constraints like bit sizeof the data, integrators and gain multipliers. CTL 125 provides adown-converted received signal 126. The latter is provided to otherportions (not shown) of receiver 15 for recovery of the data conveyedtherein and, in accordance with the principles of the invention, is alsoprovided to lock detector 200 (described below), which provides a locksignal 141 as a function of the down-converted received signal 126.

Before describing lock detector 200, reference should be made to FIG. 4,which shows an illustrative embodiment of CTL 125. CTL 125 includesdelay/Hilbert filter element 120, complex multiplier 150, phase detector155, loop filter 160, combiner (or adder) 165, numerically controlledoscillator (NCO) 170 and sine/cosine (sin/cos) table 175. It should benoted that other carrier tracking loop designs are possible, as long asthey achieve the same performance.

Delay/Hilbert filter element 120 includes a Hilbert filter and anequivalent delay line that matches the Hilbert filter processing delay.As known in the art, a Hilbert Filter is an all-pass filter thatintroduces a −90° phase shift to all input frequencies greater than 0(and a +90° degree phase shift to negative frequencies). The Hilbertfilter allows recovery of the quadrature component of the output signal116 from BPF 115. In order for the CTL to correct the phase and lock tothe ATSC IF carrier both the in-phase and quadrature components of thesignal are needed.

The output signal 121 from delay/Hilbert filter element 120 is a complexsample stream comprising in-phase (I) and quadrature (Q) components. Itshould be noted that complex signal paths are shown as double lines inthe figures. Complex multiplier 150 receives the complex sample streamof signal 121 and performs de-rotation of the complex sample stream by acalculated phase angle. In particular, the in-phase and quadraturecomponents of signal 121 are rotated by a phase. The latter is providedby signal 176, which represents particular sine and cosine valuesprovided by sin/cos table 175 (described below). The output signal fromcomplex multiplier 150, and for that matter CTL 125, is down-convertedreceived signal 126, which represents a de-rotated complex samplestream. As can be observed from FIG. 4, down-converted received signal126 is also applied to phase detector 155, which computes any phaseoffset still present in the down-converted signal 126 and provides aphase offset signal indicative thereof. This computation can beperformed with a “I*Q” or a “sign(I)*Q” function. The phase offsetsignal provided by phase detector 155 is applied to loop filter 160,which is a first order filter with proportional-plus-integral gains.Ignoring for the moment combiner 165, the loop filtered output signalfrom loop filter 160 is applied to NCO 170. The latter is an integrator,which takes as an input signal a frequency, and provides an outputsignal representative of phase angles associated with the inputfrequency. However, in order to increase the acquisition speed, the NCOis fed a frequency offset, F_(OFFSET), corresponding to F_(PILOT) (thefrequency of the carrier pilot tone present in the VSB signal), which isadded to the loop filter output signal via combiner 165 to provide acombined signal to NCO 170. NCO 170 provides an output phase anglesignal 171 to sin/cos table 175, which provides the associated sine andcosine values to complex multiplier 150 for de-rotation of signal 121 toprovide down-converted received signal 126. It should also be noted, andas mentioned earlier, that loop filter 160 may also be used to provide alock signal 127, which is shown in dashed line form on FIG. 4.

As noted above, we have observed that when a carrier tracking loop (CTL)of an ATSC VSB receiver is actually locked, the carrier pilot present inthe received ATSC VSB signal creates a DC offset in the CTL outputsignal (the down-converted received signal). As such, we have realizedthat this DC offset can be used to determine the locked condition of theCTL. In particular, the DC offset can be recovered by averaging the CTLoutput signal. Further, since the carrier pilot power is known to be11.3 dB below the signal power, an estimate of what the DC offset shouldbe can be derived from the signal power of the CTL output signal. Assuch, a decision device can then be used to determine whether the CTL isin a locked condition or an unlocked condition as a function of theestimated value of the DC offset and the actual value of the DC offset.Indeed, this technique is applicable to any modulation system for whicha carrier pilot is included in the transmitted signal and the receiverdemodulates a corresponding received signal by down converting thereceived carrier pilot to DC.

In view of the above, and in accordance with the principles of theinvention, lock detector 200 of FIG. 3 provides a lock signal 141 as afunction of the down-converted received signal 126, where the locksignal 141 is representative of either a locked condition or an unlockedcondition of CTL 125. Illustratively, lock detector 200 utilizes the DCoffset of the carrier pilot present in the down-converted receivedsignal 126 for determining the locked or unlocked condition. Referenceto FIG. 5 shows an illustrative embodiment of lock detector 200. Lockdetector 200 comprises averaging (avg.) filter 135, DC offset estimator130 and comparator 140. The down-converted received signal 126 isapplied to avg. filter 135 and DC offset estimator 130. The former is afilter of relatively low bandwidth (approximately 1 KHz (thousands ofhertz) or less) that averages the down-converted received signal 126 andprovides DC offset signal 136, which is representative of whether or notthe carrier pilot is present. Since the carrier pilot power is known tobe 11.3 dB below the signal power, DC offset estimator 130 estimateswhat the value of the DC offset should be from the down-convertedreceived signal. Illustratively, this estimated value is derived fromthe power-level of the down-converted received signal. For example, DCoffset estimator 130 estimates the DC offset by first determining thepower level of the down-converted received signal from the followingequation:

$\begin{matrix}{P_{VSB} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}\left( r_{i}^{2} \right)}}} & (1)\end{matrix}$where r_(i) is a sample of the down-converted received signal at time iand N is the number of samples used in the estimation. Once a value forP_(VSB) is determined, DC offset estimator 130 then determines theestimated DC offset, as represented by estimated DC offset signal 131,which is proportional to the square root of P_(VSB) by, e.g., addressinga predefined lookup table (not shown) that maps individual values ofP_(VSB) to respective estimated DC offset values.

As can be observed from FIG. 5, the DC offset signal 136 and theestimated DC offset signal 131 are applied to a decision device such ascomparator 140. The latter is used to determine whether CTL 125 is in alocked condition or an unlocked condition as a function of the estimatedvalue for the DC offset and the actual value of the DC offset.Illustratively, comparator 140 implements a simple function thatperforms a comparison based on a predefined threshold. For example,comparator 140 first calculates a value, s, which is the absolute valueof the difference between the DC offset and the estimate of the DCoffset, or:s=|DC offset−estimate of the DC offset|.   (2)

If the value of s is within a specified threshold, then CTL 125 islocked; otherwise CTL 125 is unlocked. For example, when CTL 125 is, infact, locked for a period of time, the estimate of the DC offset valueapproximates the actual DC offset, i.e., the value of s will approximatezero. However, as CTL 125 begins to drift, the actual DC offset willbegin to drop in value—thus, causing the value of s to increase. This isillustrated in Table 1 of FIG. 6, which shows the value of lock signal141 as a function of the comparison between DC offset signal 136 andestimated DC offset signal 131. If s is less than or equal to thepredefined threshold, than lock signal 141 is set to represent a lockedstate, e.g., a logic level of “1.” Otherwise, lock signal 141 is set torepresent an unlocked state, e.g., a logic level of “0.” An illustrativethreshold value could be a fraction of the estimated DC offset value,e.g., one eighth of the estimated DC offset value.

Turning now to FIG. 7, an illustrative flow chart in accordance with theprinciples of the invention is shown. In step 305, receiver 15 downconverts a received signal to generate a down-converted signal. In step310, receiver 15 averages the down-converted signal to provide anaverage signal (e.g., the above-described DC offset signal). In step315, receiver 15 generates an estimate of the average signal (e.g., theabove-described estimated DC offset signal) from the down-convertedsignal. In step 320, receiver 15 determines the value of a parameter s(e.g., equation (2), above). In step 325, receiver 15 compares the valueof s to a predetermined threshold. If the value of s is greater than thepredetermined threshold, then receiver 15 determines that the CTL isunlocked in step 335. On the other hand, if the value of s is less than,or equal to, the predetermined threshold, then receiver 15 determinesthat the CTL is locked in step 330.

It should be noted that since lock detector 200 is placed outside of thecarrier tracking loop, lock detector 200 is less dependent on thecarrier tracking loop parameters and bandwidth. Advantageously, theaveraging filter and DC offset estimator bandwidth can be set topractically eliminate the noise influence on the lock detector withoutaffecting the tracking ability of the carrier tracking loop, making lockdetector 200 more reliable.

Another embodiment in accordance with the principles of the invention isshown in FIG. 8. The embodiment of FIG. 8 is similar to that shown inFIG. 3 except that an additional lock detector 250 is shown. As notedearlier, CTL 125 may provide a lock signal 127 from, e.g., loop filter160. Further, as described above, lock detector 200 provides lock signal141. In accordance with a feature of the invention, lock detector 250uses a combination of lock signal 127 and lock signal 141 for generatinganother lock signal 251 as indicative of whether or not CTL 125 iseither locked or unlocked. For example, lock detector 250 may simplylogically “OR” the two signals together. It should be noted thatalthough shown as a separate element, lock detector 250 may also be apart of lock detector 200, which would then be modified to also receivelock signal 127 from CTL 125. In the embodiment exemplified by FIG. 8,lock signal 127 reflects a short term behavior of the CTL (highbandwidth) while lock signal 141 reflects a long term behavior of theCTL (low bandwidth). As such, lock signal 127 and lock signal 141 can beused together to indicate the presence of impulse noise, thermal noiseor phase noise, in the received signal data.

Although the inventive concept was described above in the context of anATSC VSB television receiver, the inventive concept is not so limitedand applies to any receiver that down-converts a received signal thatincludes a pilot carrier. Turning now to FIG. 9, another embodiment inaccordance with the principles of the invention is shown. A portion 40of a receiver, e.g., receiver 15 of FIG. 2, includes carrier trackingloop (CTL) 50 and lock detector 55. A received signal 49, including apilot carrier signal, is applied to CTL 50, which providesdown-converted output signal 51 to lock detector 55. As can be observedfrom FIG. 7, down-converted output signal 51 is also available forprocessing by other portions (not shown) of the receiver for recovery ofdata conveyed therein. In accordance with the principles of theinvention, lock detector 55 determines if CTL 50 is in a lockedcondition or an unlocked condition as a function of down-convertedoutput signal 51. Lock detector 55 provides lock signal 56, which isrepresentative of the locked or unlocked condition of CTL 50.Illustratively, CTL 50 and lock detector 55 may include elements similarto those shown and described above with respect to FIGS. 3, 4 and 5 butare not so limited.

FIG. 10 shows another illustrative embodiment in accordance with theprinciples of the invention. The embodiment of FIG. 10 is similar to theembodiment of FIG. 9, described above, except for the addition of finallock detector 60. In particular, in this embodiment a lock condition isrepresented by a value of signal 61. The latter is provided by finallock detector 60 which determines whether or not an actual lockcondition is present as a function of at least both lock signal 56(provided from lock detector 55) and lock signal 52, which is providedfrom CTL 50 by, e.g., the earlier noted loop filter (e.g., loop filter160 of FIG. 4). For example, final lock detector 55 indicates a lockcondition only if both lock signal 56 and lock signal 52 arerepresentative of a lock condition.

It should also be noted that groupings of components for particularelements described and shown herein are merely illustrative. Forexample, although FIG. 4 shows a Hilbert filter internal to the carriertracking loop, this is not required and, e.g., the Hilbert filter couldhave been shown in FIG. 3 and described as external to the carriertracking loop.

As such, the foregoing merely illustrates the principles of theinvention and it will thus be appreciated that those skilled in the artwill be able to devise numerous alternative arrangements which, althoughnot explicitly described herein, embody the principles of the inventionand are within its spirit and scope. For example, although illustratedin the context of separate functional elements, these functionalelements may be embodied on one or more integrated circuits (ICs).Similarly, although shown as separate elements, any or all of theelements may be implemented in a stored-program-controlled processor,e.g., a digital signal processor, which executes associated software,e.g., corresponding to one or more of the steps shown in FIG. 7.Further, although shown as elements bundled within TV set 10, theelements therein may be distributed in different units in anycombination thereof. For example, receiver 15 of FIG. 2 may be a part ofa device, or box, physically separate from the device, or box,incorporating display 20, etc. Also it should be noted that although thedown-converted received signal was illustratively averaged, otherstatistical functions may be used along with respective modificationsto, e.g., step 315 of FIG. 7. It is therefore to be understood thatnumerous modifications may be made to the illustrative embodiments andthat other arrangements may be devised without departing from the spiritand scope of the present invention as defined by the appended claims.

1. Apparatus for use in a receiver, the apparatus comprising: a carrier tracking loop (CTL) for down-converting a received signal to provide a down-converted received signal; a CTL lock detector for detecting, as a function of the down-converted received signal, whether the CTL is in a locked condition or an unlocked condition, wherein the CTL lock detector provides a lock signal representative of the locked condition or the unlocked condition; and another lock detector for use in determining the locked condition of the CTL as a function of the lock signal and another signal from the CTL that is also representative of a locked condition of the CTL.
 2. The apparatus of claim 1, wherein the CTL lock detector comprises: an averaging filter for averaging the down-converted received signal to provide an average signal; a signal estimator responsive to the down-converted received signal for providing an estimated signal, which is representative of an estimate of the average signal; and a decision device for providing a lock signal as a function of the estimated signal and the average signal.
 3. The apparatus of claim 2, wherein the decision device is a comparator that compares the estimated signal to the average signal for determining that the CTL is either in the locked condition or the unlocked condition.
 4. The apparatus of claim 1, wherein the received signal is an ATSC (Advanced Television Systems Committee) VSB (Vestigial Sideband) modulation signal.
 5. Apparatus for use in an ATSC (Advanced Television Systems Committee) VSB (Vestigial Sideband) receiver, the apparatus comprising: a carrier tracking loop responsive to a received ATSC VSB signal for providing a down-converted ATSC signal; and a carrier tracking loop lock detector for determining if the CTL is in a locked condition or an unlocked condition as a function of the down-converted ATSC signal; wherein the carrier tracking loop lock detector comprises: an averaging filter for averaging the down-converted ATSC signal to provide an average value of the down-converted ATSC signal; an estimator responsive to the down-converted ATSC signal for providing an estimated value of the average value; and a comparator for determining if the CTL is in a locked condition or an unlocked condition as a function of a comparison between the average value and the estimated value.
 6. The apparatus of claim 5, wherein the carrier tracking loop lock detector provides a lock signal representative of the locked condition or the unlocked condition.
 7. The apparatus of claim 6, further including another lock detector for use in determining the locked condition of the CTL as a function of the lock signal and another signal from the CTL that is also representative of a locked condition of the CTL.
 8. A method for use in an ATSC (Advanced Television Systems Committee) VSB (Vestigial Sideband) receiver, the method comprising the steps of: using a carrier tracking loop for processing a received ATSC VSB signal to provide a down-converted ATSC signal; and determining if the CTL is in a locked condition or an unlocked condition as a function of the down-converted ATSC signal; wherein the determining step includes the steps of: averaging the down-converted ATSC signal to provide an average value of the down-converted ATSC signal; estimating, from the down-convened ATSC signal, the average value; and determining if the CTL is in a locked condition or an unlocked condition as a function of a comparison between the average value and the estimated value.
 9. The method of claim 8, further comprising the step of providing a lock signal representative of the locked condition or the unlocked condition.
 10. The method of claim 9, further comprising the step of determining the lock condition of the CTL as a function of the lock signal and a signal from the CTL that is also representative of a locked condition of the CTL. 